Define the per-project category and difficulty at the template level,
instead of manually tagging the project pages. While doing this, prefix
the tags with "category:" and "difficulty:" to make them easier to
understand.

Move existing project definitions from projects/gsoc_2011/ to
projects/project/ .
The goal for this reorganization is to remove any knowledge of the projects
classification from the file hierarchy: the classification goes into tags,
and projects indexes automatically list projects based on such tags.
Also, the current gsoc_2011 name was wrong anyway, because GSoC 2011 has
already concluded and projects would have had to move to a gsoc_2012 directory
anyway.
Lastly, yes, "projects/project/*" is slightly redundant. But I want to keep
the project lists from the projects "database" clearly separated.
This is as proposed in www@.